Radionuclides are often referred to by chemists and biologists as radioactive isotopes or radioisotopes, and play an important part in the technologies that provide us with food, water and good health. However, they can also constitute real or perceived dangers.

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Naturally occurring radionuclides fall into three categories: primordial radionuclides, secondary radionuclides and cosmogenic radionuclides. Primordial radionuclides originate mainly from the interiors of stars and, like uranium, are still present because their half-lives are so long that they have not yet completely decayed. Secondary radionuclides are radiogenic isotopes derived from the decay of primordial radionuclides. They have shorter half-lives than primordial radionuclides. Cosmogenic isotopes, such as carbon-14, are present because they are continually being formed in the atmosphere due to cosmic rays. Despite their "short" half-lives, they are found in nature because their supply is always being replenished.

Artificially produced radionuclides can be produced by nuclear reactors, particle accelerators or by radionuclide generators:

Radioisotopes produced with nuclear reactors exploit the high flux of neutrons present. The neutrons are used to activate elements placed within the reactor. A typical product from a nuclear reactor is thallium-201.

Particle accelerators such as cyclotrons accelerate particles to bombard a target to produce radionuclides. Cyclotrons are used to accelerate protons at a target to produce positron emitting radioisotopes e.g. fluorine-18.

Radionuclide generators contain a parent isotope that decays to produce a radioisotope. The parent is usually produced in a nuclear reactor. A typical example is the technetium-99m generator used in nuclear medicine. The parent produced in the reactor is molybdenum-99.

Trace radionuclides are those that occur in tiny amounts in nature either due to inherent rarity, or to half-lives that are significantly shorter than the age of the Earth. Synthetic isotopes are not naturally occurring on Earth, but they can be created by nuclear reactions.

Radionuclides are used in two major ways: for their chemical properties and as sources of radiation.
Radionuclides of familiar elements such as carbon can serve as tracers because they are chemically very similar to the non-radioactive nuclides, so most chemical, biological, and ecological processes treat them in a near identical way. One can then examine the result with a radiation detector, such as a geiger counter, to determine where the provided atoms ended up. For example, one might culture plants in an environment in which the carbon dioxide contained radioactive carbon; then the parts of the plant that had laid down atmospheric carbon would be radioactive.

In medicine, radioisotopes are used for diagnosis, treatment, and research. Radioactive chemical tracers emitting gamma rays can provide diagnostic information about a person's internal anatomy and the functioning of specific organs. This is used in some forms of tomography: single photon emission computed tomography and positron emission tomography scanning.

Radioisotopes are also a promising method of treatment in hemopoietic forms of tumors, while the success for treatment of solid tumors has been limited so far. More powerful gamma sources are used to sterilise syringes and other medical equipment. About one in two people in Western countries are likely to experience the benefits of nuclear medicine in their lifetime.

Environmentally, radionuclides are used to trace and analyze pollutants, to study the movement of surface water, and to measure water runoffs from rain and snow, as well as the flow rates of streams and rivers.
Natural radionuclides can be used in archaeology and in paleontology to measure ages. When radioactive carbon, for example, is in the atmosphere, it rapidly becomes separated from its decay products. Once it is bound up in a solid, such as wood or paper, its decay products must remain in place. So by measuring how much of these decay products has accumulated, one can estimate the time when the carbon was captured into solid form.

If radionuclides are released into the environment, through accident, poor disposal, or other means, they can potentially cause harmful effects of radioactive contamination. They can also cause damage if they are excessively used during treatment or in other ways applied to living beings. This is called radiation poisoning. Radionuclides can also cause malfunction of electrical devices.